arch: m68k: 68360: config: Remove unused function

Remove the function BSP_set_clock_mmss() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>